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Owing to the generality of the question I am interested merely in key problems so that I could do further reading.

As I understand man-made energy storages are significantly less efficient than biological ones. I would like to focus on quickly available energy for small systems (human scale or smaller) - after all we can say that hoards of coal are a pretty nice energy reservoir.

For example, a robotic bee:

https://www.youtube.com/watch?v=lJCMIsLuGpg

What can we see here? A wire - and I think we cannot expect to have a long lasting independent robotic bee with the current technology due to inefficiency of our energy storage.

How do biological systems store the energy then? https://en.wikiversity.org/wiki/Energy_Storage_in_Biological_Systems

Living organisms use two major types of energy storage. Energy-rich molecules such as glycogen and triglycerides store energy in the form of covalent chemical bonds. Cells synthesize such molecules and store them for later release of the energy. The second major form of biological energy storage is electrochemical and takes the form of gradients of charged ions across cell membranes. This learning project allows participants to explore some of the details of energy storage molecules and biological energy storage that involves ion gradients across cell membranes.

To be more specific let me give an example: why cannot we have a machine that would have an input for carbohydrates, a converter to say glucose and then ATP which would finally break down and release the needed energy?

Why is it not viable? Do we lack technology to do this? Which step is problematic? Are we far from having biological-like storages?

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    $\begingroup$ We built it. It's called the methane fuel cell. It can run in reverse. $\endgroup$ – Joshua Nov 17 '16 at 2:59
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The activation energy of breaking these covalent bonds is high. Biological systems use phosphorylases to couple ATP to glycogen such that the high energy phosphate bonds then supply some of their energy to break the covalent glycosidic bonds in glycogen here

So problem 1, high activation energy.

Secondly, how are you going to store the energy in the machine? Conventional electrochemical cells use something like what was written about membrane potential. Their energy is accessible as it is in a form of an electrical potential. When you expend energy to break storage molecules into glucose, there's not much for you to do on the glucose other than to burn it. Burning it is extremely inefficient and makes up a huge amount of energy loss to wasted heat. The other way would be to oxidize it. You'll need something to accept the electrons (NADH and FADH in biological systems), something to overcome the high activation energy of oxidizing the bonds at every stage (more ATP coupling), something to harness the high-energy electrons from glucose (electron transport chain/chemiosmosis). In all, it is too complicated to be replicated efficiently.

So problem 2, there are many complicated steps involved.

This is why only simpler versions of the above exist. Power to gas shows us the splitting of H2O into H2 and O2. This is akin to only one stage of bond-breakage in respiration, and is simple enough to carry out (through electrolysis). H2O donates and another H2O accepts the electron. Energy is supplied as a electrical potential in the circuit instead of ATP coupling from enzymes.

Last problem is, there really isn't a need to store energy in other forms than an already-existing energy storage molecule. Most of our energy storage is in the form of carbon compounds, and electricity. Carbon compounds are high energy density, and electricity is easy to manipulate. It is perhaps trivial to the world to attempt condensing table sugar (glucose) into starch. It is not efficient enough to be impactful. Biological systems need to do this because storage is on a scale comparable to the cell body, and directly relates to survival. High blood glucose in animals might lead to hyperglycemia, triose phosphate (product of respiration) is used for lipid formation and in turn building of cell membranes, glucose molecules are used as monomers in the production of cellulose, aka cell wall in plants, (a heck ton of other metabolic pathways exist), etc.

This link shows us the different ways of energy storage in our world. We're not limited by biological constraints, so even compressing H2 into a liquid, will be more efficient than interconverting that into other forms of chemical energy, if you're talking about for pure "storage"'s sake.

I highly recommend reading up on photosynthesis, to see how biological systems harness energy from the sun, and store that into covalent bonds between CO2 molecules. More specifically you can look up Krebs Cycle.

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  • $\begingroup$ A 'high-energy' phosphate bond is a 'source' of Gibbs free energy, not of bond energy. Furthermore, there is no 'high-energy' bond (in the Lippmann sense) whatsoever in glucose-1-phosphate. (The OP has stated that the machine will 'store' free energy in the form of ATP, that is by keeping the ATP + H20 = ADP + Pi reaction far from equilibrium) $\endgroup$ – user1136 Oct 29 '16 at 10:30
  • $\begingroup$ Yes. Gibbs free energy when it comes to spontaneity of the reaction. Thank you for your clarification. The "high energy" bond was referring to the high energy electrons shuttled by the reduced coenzymes as they travel down the ETC. Probably bad phrasing. $\endgroup$ – Liu Tianyi Oct 29 '16 at 17:59
  • $\begingroup$ The OP's question is a bit broad and vague, so let's focus on his example, an insect. Bees (let alone butterflies) can fly miles with the chemical energy stored in their bodies. Why can't small mechanical flying objects? The reason is clearly not storage (as opposed to what the OP's title suggests); we can store sugar and fat perfectly well. But using it for mechanical work as efficiently as a biological system does seems impossible for now; we tend to explode and burn stuff, which is terrible. $\endgroup$ – Peter A. Schneider Nov 17 '16 at 10:24

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